1. Field of the Invention
This invention relates to nonlinear optical devices and more particularly to devices for doubling the frequency of electromagnetic radiation propagating therethrough.
2. Description of Related Art
Optical digital data storage devices, such as compact discs, are now commonly used for many purposes, such as for the storage of digitized video, audio and computer data. Typically the data is written onto and read from such discs by means of a light emitted by a semiconductor laser diode. The light generated by available semiconductor laser diodes generally falls within the lower end of the electromagnetic frequency spectrum (i.e. red or infrared). The use of higher frequency light (i.e. at the blue end of the spectrum) to read and write the optically stored data would result in greatly increased storage density.
Unfortunately, practical higher frequency semiconductor lasers are not available. The only blue light producing lasers are large gas lasers which are unsuitable for use in compact and inexpensive optical storage read/write devices.
One way of compactly producing blue light would be to convert the infrared light emitted by readily available semiconductor laser diodes to blue light. The frequency of blue radiation is twice that of infrared radiation. Thus, a device capable of doubling the frequency of infrared radiation has considerable commercial potential.
A number of nonlinear optical devices capable of performing frequency doubling are known. Typically these doubling devices include bulk materials or stacks of nonlinear crystals configured to perform frequency doubling by second harmonic generation of a fundamental frequency.
A particularly effective doubling device is a nonlinear optical waveguide. As a fundamental frequency light wave propagates through the waveguide, the nonlinear optical effect causes the generation of the second harmonic frequency light wave. Such optical waveguides can efficiently effect frequency doubling only if the fundamental and doubled waves remain substantially in matched phase with each other as they propagate through respective portions of the waveguide. If the two waves do not remain in phase, interference effects will cause attenuation of the second harmonic.
It is very difficult to produce waveguide doublers having the precision dimensional and physical properties required for accurate phase matching. Further, waveguide doublers with fixed properties are incapable of responding to changing ambient conditions which would affect the efficiency of operation. For example, changes in temperature will affect the refractive indices of the materials used in the doublers. It is therefore desirable to provide a waveguide doubler which can be actively "tuned" to allow for manufacturing tolerances and changes in ambient conditions.
Proposals for active phase matching of waveguides have been made in, for example, U.S. Pat. No. 4,427,260 to Puech et al, which issued on 24 Jan. 1984, and in the article "Electric Field Tuning of Second-Harmonic Generation in a Three-Dimensional LiNbO.sub.3 Optical Waveguide", Applied Physics Letters, 34(1), 1 Jan. 1979. These proposals aim to achieve phase matching by electro-optically tuning the materials forming the waveguides. However, the proposed electro-optical tuning approaches will achieve only relatively small changes in the indices of refraction of the waveguides and, consequently, will achieve only relatively small changes in phase matching.